Explain why cells in the same organism can have very different structures even though they all contain the same DNA.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During development, a single fertilised egg divides repeatedly by mitosis to produce all the cells of an organism. Despite containing identical genetic information, cells such as erythrocytes, goblet cells and neurones differ dramatically in their structure and function.
Model answer (4 marks)
All cells contain the same genes but only certain genes are expressed in a given cell type.
Gene expression leads to transcription of specific mRNA molecules in that cell type.
The mRNA is translated to produce specific proteins (e.g. structural proteins or enzymes) characteristic of that cell type.
This selective gene expression is the basis of cell differentiation, resulting in cells with different structures adapted to their function.
Gene expression leads to transcription of specific mRNA molecules in that cell type.
The mRNA is translated to produce specific proteins (e.g. structural proteins or enzymes) characteristic of that cell type.
This selective gene expression is the basis of cell differentiation, resulting in cells with different structures adapted to their function.
Examiner tips
- Use the word ‘expressed’ to show understanding of gene regulation.
- Show the flow from DNA → mRNA → protein to link structure and function.
- Mention that only a subset of genes is active in each cell type.
- Keep the answer concise – 4 points, one sentence each.
Common mistakes
- Confusing ‘genes’ with ‘proteins’ – students write ‘different DNA’ instead of ‘different gene expression’.
- Failing to mention the mRNA step or the translation to proteins.
- Writing a long paragraph instead of four clear points.
Mark scheme (4 marks)
- All cells contain the same genes but only certain genes are expressed (switched on) in a given cell type
- Gene expression leads to transcription of specific mRNA molecules in that cell type
- The mRNA is translated to produce specific proteins (e.g. structural proteins or enzymes) characteristic of that cell type
- This selective gene expression is the basis of cell differentiation, resulting in cells with different structures adapted to their function
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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